Question: In geometry class, the girl to boy ratio is $9$ to $4$. If there are $20$ boys, how many girls are there?
Answer: A ratio of $9$ girls to $4$ boys means that there is a group of $9$ girls for every group of $4$ boys. If there are $20$ boys, then there are $5$ groups of $4$ boys each. Because we know the ratio is $9:4$ , there must be $5$ groups of $9$ girls each. There is a total of $45$ girls in geometry class.